1. Field of the Invention
The present invention relates in general to a limited slip differential for a vehicle and, in particular, to an electrically controllable biasing differential.
2. Description of the Prior Art
Conventional differential mechanisms consist of a set of bevel gears coupled between two half-shafts of a drive axle. Such a drive axle has the advantages over a solid axle that the wheels of the vehicle can travel at different speeds and equal driving force can be applied to the driving wheels. However, under certain conditions, this conventional differential has a serious deficiency. For example, if one drive wheel is on a slippery surface, such as ice or mud, that wheel will slip and spin because its tire can not grip the road. Consequently, the slipping wheel can apply very little driving torque to move the car. The opposite drive wheel, which well may be on a surface that gives good adhesion, can apply no more driving torque than the spinning wheel because the differential delivers only an equal amount of torque to both wheels. Thus, the total driving force can never be more than twice the amount applied by the wheel with the poorest road adhesion. Traction is also adversely effected, especially during hard driving, by other conditions that unbalance the weight on the driving wheels. When driving at high speed around a curve, the weight is transferred from the inside wheel to the outside wheel. Hard acceleration coming out of a turn can then cause the inside wheel to spin because it has less weight on it and therefore less road adhesion. Similarly, during any hard acceleration there is propeller shaft reaction torque on the rear axle assembly. When one wheel is partially unloaded and looses part of its traction capability, the loss is not offset by gain on the opposite side because the total can only be twice that of the wheel with the lesser capability.
The limited slip differential was designed to improve the traction of a vehicle under adverse traction conditions by allowing the differential to transmit torque to the axle shafts in unequal amounts without interfering with the differential action on turns. The most common limited slip differential is the friction type, which has clutch assemblies mounted between the two side gears and the differential case. In a conventional differential, the side gears and the axle shafts to which they are splined always turn freely in the case. The added clutches provide a means of transferring torque from the faster spinning (usually slipping) wheel to the slower spinning (usually better adhesion) wheel. Typically, there are two clutch packs, each of which is comprised of disks that are splined to the side gear, and plates that are tanged to fit into the differential case. Thus, the disks rotate with the side gear and the plates rotate with the case. The clutches are applied or actuated by two forces. One force is applied by springs compressed between the two side gears, which push the side gears apart, towards the case, and thus keep the plates and disks in contact with each other. This force is relatively constant and preloads the clutches. The other force results from the tendency of the pinions and side gears to push themselves apart as they turn. This force is applied through the side gears and increases the pressure on the plates and disks. This force becomes greater as the driving torque transmitted from the pinions to the side gears increases and is therefore a variable force.
The typical limited slip differential has a design limit on the amount of torque transfer from the faster to the slower wheel, so that the torque on the wheel with good traction is about two and one half times that of the wheel with poor traction. From the above description, several shortcomings of the common limited slip differential are apparent:
(1) During turning maneuvers, torque is transferred to the inside wheel at a rate generally proportional to the driving torque. This results in a tendency to under steer.
(2) Under conditions where one driving wheel is on a very slippery surface while the other has good traction, the amount of torque that can be transferred is very limited, essentially determined by the preload spring force on the clutch packs.
It is the intent of this invention to overcome these shortcomings by providing an externally controllable limited slip differential whose clutch actuating force is not dependent on preload springs or side gear separating forces caused by drive line torque, but rather is provided by hydraulic pressure. The hydraulic pressure is generated by an electrically actuated ball ramp mechanism, which presses on a secondary piston. This pressure may be regulated as necessary to adjust the differential from zero to full locking or full biasing, as driving needs dictate.
The present invention concerns an electrically controllable biasing differential, which utilizes a multi-disk clutch to selectively bias the differential. The clutch pack is mounted within a differential case half and the clutch disks are alternately splined to a side gear and the case half thereby providing resistance to relative rotation of the left and right output shafts of the differential.
The primary feature of this invention is the unique way of loading the clutch pack to connect one side gear to the differential case. The loading mechanism consists of two separate sub-systems:
An electric coil and mechanical ball ramp mechanism.
A hydraulic piston mechanism.
The electric coil and mechanical ball ramp mechanism provides an axial force by utilizing the rotational displacement of two ball ramp races. When no torque bias is required, both ball ramps are free to rotate with the differential case. When the coil is energized, it provides a rotational resistance to one ball ramp race. The relative rotation between ball ramp races provides axial displacement of the unrestrained ramp race. This displacement could be used to load the differential clutch pack; however, the force generated is not sufficient to provide acceptable performance.
A simple hydraulic piston mechanism is used to amplify the axial force generated by the electric coil and mechanical ball ramp mechanism. The hydraulic system consists of an annular primary piston in contact with the clutch pack, and a multiple set of secondary pistons attached to the unrestrained ball ramp race. Hydraulic fluid fills the cavity between the primary and secondary pistons. This system provides force multiplication proportional to the surface areas of the respective piston faces.
Biasing of the differential is proportional to the torque drag applied across the ball ramp mechanism. As current flow is increased to the coil, the ball ramp races rotate relative to each other, the secondary pistons move axially to pressurize the piston cavity, and the primary piston applies a load to the clutch pack. Decreasing current flow effectively reverses the process.
The frictional drag associated with both mechanisms is relatively low. This minimizes the hysterysis bias variation as current to the coil is increased or decreased.